Generally, an image forming apparatus controls a plurality of conveying units so as to form a certain amount of bending (loop) in a recording medium. By providing the loop in the recording medium, for example, the skewing of the recording medium during conveyance can be suppressed.
Such an electrophotographic type image forming apparatus performs control for passing the recording medium through a transferring part, while keeping a fixed amount of the loop formed in the recording medium. However, the sheet holding force of the transferring part is decreased as the trailing end of the recording medium passes through the plurality of transferring parts, so that the trailing end of the recording medium vibrates at the moment when the loop is released. This is the main cause of disturbance of an unfixed toner image and thus image defects.
In order to solve this problem, there is proposed a method in Japanese Patent Laid-Open No. 2003-316184, which controls fixing and conveying speeds so as to increase or decrease the loop amount in response to the arrival of the trailing end of the recording medium at a predetermined position on the upstream side of the transfer position in the conveying direction.
However, there arises a new problem that occurrence of erroneous detection of the loop amount increases, as the path from the transferring part toward the fixing part is shortened in accordance with miniaturization of the image forming apparatus in recent years.
The invention described in Japanese Patent Laid-Open No. 2003-316184 is an excellent invention in that the loop amount is controlled so as to be increased or decreased on the basis of the trailing end of the recording medium as a reference. Specifically, the control is effective to suppress vibration of the trailing end of the recording medium. However, the loop amount may become excessive at the trailing end of a sheet in a constitution in which the path between the transferring part and the fixing part is further shortened. When the loop amount becomes excessive, the sheet may be pressed into the transferring part, resulting in a possibility of an image defect being caused.
Further, the pressure of a nip part between a fixing roller and a pressing roller in the fixing part is generally set to be different between the central part and the end part of the rollers. This is a measure for preventing wrinkles of the sheet from being caused after fixation. The pressure difference in the nip part causes a difference in the loop amount between the central part and the end part in the fixing part. In the constitution in which the path between the transferring part and the fixing part is short, such difference in the loop amount affects color slurring in the transferring part.
Consequently, there is a room for improvement in suppressing image defects, in order to attain further miniaturization of the image forming apparatus.